Rotary joint



Feb. 4, 1969 R. M. GOODMAN, JR r3,426,309

ROTARY JOINT Filed Nov. 7, 1966 Q Q ww Nw n a 9W uw QM VII., fill ww Qw .llh\\\\\\x`\f/. .ww www m \\`\\mlllllll||l| wh vh N mw Y @l mi I 0m. md AW? QN N QQ\ N QU J /wwd @NJ w ,w Q I wm l m s Q mm QM. U. Q Wk I l wlzmldgv www, n w m Q ww w vv, u.\\ WW %\v1 WR \WN\\ NQ\ QQ\ Nw www@ u Xwm United States Patent O 3,426,309 ROTARY JOINT Robert M. Goodman, Jr., Marietta, Ga., assignor to Scientific-Atlanta, Inc., Atlanta, Ga., a corporation of Georgia Filed Nov. 7, 1966, Ser. No. 592,402 U.S. Cl. 339-8 Int. Cl. H011' 39/08, 39/30 Claims ABSTRACT OF THE DISCLOSURE Disclosure This invention relates to high speed electrically conducting rotary joints and, in particular, a coaxial rotary joint through which a broad band of frequencies (for example, DC through 12 gc.) can be transmitted.

This invention also relates to improved apparatus for sealing a uid slip ring path for DC signals where the fluid is typically mercury.

In the design of electrical rotary joints of the prior art, attention in some instances has been paid mostly to mechanical considerations involved in designing the joint. For instance, Patent 2,424,545 granted to Francis N. Bard `discloses a coaxial rotary joint of good mechanical construction and aptly suited for the passage of DC and low'frequency signals. However, the Bard structure is not well suited for the passage of high frequency signals, for example those above 1000 mc., since the ratio of the inner conductor diameter to the inside diameter of the outer conductor is not maintained constant from the input terminals to the output terminals of the rotary joint. This ratio ydetermines the impedance of the coaxial line and any abrupt changes thereof will seriously limit the ability of the joint to pass higher frequency or RF energy etliciently. In the above-mentioned Bard patent, the ratio of the inner and outer conductor diameters of the coaxial line is abruptly changed for mechanical convenience. Hence, mechanical rather than electrical considerations are the dominant factors in the Bard rotary joint. However, the need for passing RF electrical energy through the rotary joint arises in many applications-for instance, in barrier and intrusion detection systems.

Thus, it is a primary object of this invention to provide an improved electrical rotary .joint `for passing a wide range of frequencies, for example, DC to 12'gc.

It is another object of this invention to provide improved rotary joints for use in such applications as barrier and intmsion detection systems.

In order to efficiently passvbroad band signals through an electrical rotary joint, it has been determined in accordance with the teachings of this invention that separate (l) high frequency or RF, and (2) DC and low frequency paths must be provided in both the inner and outer conductors of a coaxial joint. Thus, it is a more detailed object of this invention to provide an improved rotary joint wherein separate paths are provided for (l) RF,

3,426,309 Patented Feb. 4, 1969 ice and (2) low frequency and DC signals in both the inner and outer conductors of the joint.

When the band pass of a rotary joint includes DC 0r low frequency signals, considerable diiculty has been encountered due to intermittent open contact and to the generation, in the rotary joint, of noise levels comparable to fthe signal level resulting in low, vari-able, and decreasing signal-to-noise ratios with continued use of the rotary joint a high rotational speeds. The use lof mercury in a DC path slip ring in a rotary joint is known as disclosed in the above-mentioned Bard patent. However, a typical shortcoming of such mercury slip ring paths arises because of the type of seal employed to retain the mercury. In particular, unacceptably high seal friction, heat and wear severely limit the usefulness of such rotary joints. When the terminals of the rotary joint are connected to coaxial cable, the rotational friction torque developed by the seals for the mercury must be resisted by torsional stress in the coaxial cable. Hence, in the coaxial applications, the amount of `friction torque which can be tolerated is further limited.

Thus, it is a further object of this invention to provide improved seals for use in sealing a DC path through a fluid slip ring where t-he duid may be mercury, for example.

A further object of this invention is to provide a rotary joint having a mercury slip ring path for DC signals where the seal for the mercury effectively retains the mercury while at the same time developing relatively 10W frictional torque.

Other objects and advantages of this invention will become apparent upon reading the appended claims in conjunction with the following detailed description and the attached drawings, in which:

FIGURE 1 shows a partial cross section view of an illustrative embodiment of the invention;

FIGURE 2 shows -a cross section view of a ball bearing assembly employed as an improved seal employed in the embodiment illustrated in FIGURE 2; and

FIGURE 3 shows in `detail the inner conductor ernployed in the illustrative embodiment of FIGURE 1.

Referring to FIGURE l, there is shown a partial cross section view of an illustrative embodiment of a coaxial rotary joint in laccordance with principles of the invention. The coaxial connector 10 is assumed to be rotating, while coaxial connector 12 is assumed to be stationary. Housing 14 is connected to coaxial connector 12 by male connector member 20. The connector 10 has integrally connected thereto a rotating outer conductor or first outer cylindrical conductor 16 and a rotary inner conductor or first inner cylindrical conductor 18.

The stationary outer conducter or second outer cylindrical conductor, as this term is employed in the claims, includes house 14 and male connector member 20. Stationary inner conductor or secondary inner cylindrical conductor 22 is integrally associated with stationary connector 12. Although the invention has been described in terms of connector 10 rotating with respect to connector 12, it would of course be o'bvious to one having ordinary skill in this art to have the connector 12 rotating while the connector 10 remains stationary, or to have -both rotating, there being relative rotation between the two connectors.

In both of the inner and outer conductors of the joint, there are provided two paths for electrical signals, one path being provided for DC or low frequency signals (for example DC to 1000 me.) `and the other path being provided =for RF signals (for example, 1000 mc. to 12 gc.). Thus, in the outer conductor, the RF signals are capacity coupled fromI rotating outer conductor 16 to male connector member 20 via gap 24. Preferably, the Width of the gap is 0.001. As can readily be seen from FIGURE l, gap 24 has an annular shape and is concentric to stationary inner conductor 22. The DC path for the outer conductor is from rotating conductor 16 through annular projection 26 and mercury slip ring 28 to stationary outer conductor 14, the projection 26 being integrally connected to rotating inner conductor 16.

In the inner conductor, the Separate RF and DC paths can be more readily ascertained by reference to FIGURE 3. The stationary inner conductor 22 has a first projection 30 therefrom. A second projection 32 extends into recess 34 of rotating inner conductor 18, recess 34 containing a bath 35 of electrically conductive fluid such as mercury. The `mercury is sealed in receptacle 34 by O-ring or sealing member 36. Thus, the DC signal of the inner condu-ctor passes from rotating -member 18 through the mercury bath to projection 32 and then to rotating inner conductor 22 through projection 30. The RF signal is capacitively coupled across gap 38 from the outer portion 40 of the recess 34 to stationary inner conductor 22. Thus, the separate paths for DC and RF signals in the inner conductor have now 'been described. Preferably, the gap 38 should -be approximately 0.001.

O-ring 36 is essentially a static seal in that the sliding velocity between projection 32 and O-ring 36 is approximately 0.5 feet per minute at 600 r.p.m. Since the sliding vel-ocity is slow, there is no heat build up and negligible torque. Thus O-ring 36 acts as a no-leak fluid seal for the mercury. O ring 36 is held in place by inwardly projecting annular portion 41 which is integrally connected to outer portion 40 of the recess 34.

The mercury slip ring 28 for the outer conductor is sealed by fball bearings 42 and 44. Reference should now be made to FIGURE 2 which shows in detail a cross section of la sealed ball bearing for use with this invention. This seal is commercially available and is manufactured by Marlin-Rockwell Co. (MRC) and a particular ball bearing which has been employed in a prototype of the invention is identified as MRC Bearing Model No. RSZZT. Although the ball lbearing is commercially available, use of this ball bearing as a mercury seal in accordance `with theteachings of this invention is thought to be novel.

In FIGURE 2, the outer ring 46 has grooves 48 and 50. Provided on inner ring 52 are grooves 54 and 56, which are preferably concentric with the bore of the ball bearing assembly. The ball bearings 58 and 60 are lubricated by an appropriate lubricant 62. Lubricant 62 is sealed Within the ball bearing assembly by seals 64 and 66. Each of these seals consists of an outer sealing member, preferably of -a soft synthetic rubber for providing xed seals at the outer grooves 48 and 50 and rotating seals at the inner grooves 54 and 56. The seals 64 and 66 also respectively include inner core members 68 and 72, preferably of steel, for strengthening the seal structure.

Because of the lubricant 62 Within the ball bearings 42 and 44, the seals at inner `grooves 54 and 56 will also be lubricated, thereby reducing the torsional friction developed by the seal. O-rings 74 and 76 are respectively yassociated with the outer and inner rings of ball bearing assembly 42 while O-rngs 78 and 80 are respectively associated with ball bearing assembly 44. O rings 74, 76, 78, and 80 act `as static seals against mercury leakage in one direction and 'again moisture in the other direction.

Ball bearings 42 and 82 provide mechanical alignment between the rotating and stationary portions of the rotary joint. O-rings 84 yand 86 are associated with ball bearing 82. Bearings 44 and 82 are secured in place by rings 88 and 90 respectively.

The remaining mechanical construction of the joint shown in FIGURE 1 is as follows: The connector 12 includes an outer member or connecting sleeve 92 threaded on the inside thereof for receiving a threaded coaxial male plug (not shown). Connector sleeve 92 is secured to the male connector member by retaining ring 94.

A ange 96 may be employed in certain applications. 75

Flange 96 is secured to connector assembly member 29 by flat head screws 98. Male connector member 20 is threaded on the outside thereof at 100, thereby securing the connection thereto of casing 14. Ball bearing assembly 42 is separated from connector assembly 20 by spacer -Washer 102.

Stationary inner conductor 22 has a projection 106 which mates With the plug connected into sleeve 92. Connecting member 108 is in electrical connection with male connector `member 20 and is secured in place lby seal 104. Thus, an electrical path from the outer conductor of the male plug connected into sleeve 92 is achieved through connecting member 108, assembly member 20, and thence to housing 14 or rotating outer conductor 16.

Having now described the structure of the invention, a brief description of the operation thereof follows. Assuming a broad band signal (extending from lDC to l2 gc, for example) is employed, the signal will be present on both the inner and outer conductors of the coaxial line. In sofar as the DC and lower frequency signals are to pass these signals. However, as to the RF signals, both concerned, only the inner or outer conductor is required the inner and outer conductors are required and, thus, the RF signal traverses the capacitive gaps 24 and 38 to achieve the desired result so far as the high frequency components of the input signal are concerned. Another factor which facilitates the passage of RF signals through the joint results from the ratio of the inner conductor diameter and the inner diameter of the outer conductor being maintained electrically constant throughout the length of the rotary joint. Hence, referring to FIGURES l and 3 going from right to left, the ratio of the diameter of rotating inner conductor 18 to the inner diameter of rotating conductor 16 is substantially the same as the ratio of the diameter of stationary inner conductor 22 to the inner diameter of rotating inner conductor 16 at the area adjacent the mercury slip ring 28. This ratio remains essentially the same at projection 30, as can be seen from FIGURE 3. A slight discontinuity is introduced into the impedance because of gap 38. The width of this gap is kept as small as possible to minimize the discontinuity.

Another slight discontinuity is introduced into the line impedance at gap 24, however, this also is kept to a minimum. Members 110 and 112 mechanically support the inner conductors 18 and 22 respectively. These members are preferably made of Teflon or the like so that electrical ratio will remain constant at these support members. To the left of support member 112, the ratio returns to that adjacent mercury slip ring 28, and thus, it can now be seen that the rotary joint according to this invention is ideally suited for the passage of RF frequencies because of the constant electrical ratio of inner conductor diameter to inside diameter of the outer conductor for the entire length of the joint.

Returning to the operation of the device, the DC and low frequency paths through the outer and inner conductors respectively occur through mercury paths 28 and 34. Thus, DC paths are provided in both of the inner and outer conductors. However, since only one of these conductors is needed for DC signals, it would be obvious to one having ordinary skill in this art to eliminate the mercury slip ring in either the inner or outer conductor in a given application. Further, it is obvious that, if a signal conductor only is employed (as opposed to a coaxial line), the sealed ball bearing assembly described hereinbefore for sealing a mercury slip ring could also be employed in the single conductor embodiment. In this instance, there would be but a single rotating conductor and a single stationary conductor with the mercury slip ring providing electrical connection therebetween.

Nut 114 is employed to hold the entire assembly together, preventing axial movement of the rotating and stationary components with respect to each other. The torque to which nut 114 is tightened affects the spacing vof gaps 24 and 38. As the nut `114 is screwed into housing 14, a force is applied to assembly 82 forcing it against clip ring 90. As the nut is further screwed into housing 14, the load is transmitted through clip ring 88, ball bearing assembly 44, and wiper 26 to ball bearing assembly 42 thereby securing assembly 42 against spacer washer 102. As the load is applied to clip ring 88, a slight movement to the left of rotating outer conductor 16 and rotating inner conductor 18 will take place due to deflections in the affected components. The width of gap 24 and of gap 38 can be adjusted appropriately by selectively varying the thickness of spacer washer 102.

Set screw 116 is removed whenever the mercury slip ring must be replenished, the position of the set Screw with respect to the slip ring not being critical.

Numerous modifications of the invention will become apparent to one of ordinary skill in the art upon reading the foregoing disclosure. During such a reading, it will be evident that this invention has provided a unique apparatus for accomplishing the objects and advantages herein stated. Still other objects and advantages, and even further modifications will be apparent from this disclosure. It is to be understood, however, that the foregoing disclosure is to be considered exemplary and not limitative, the scope of the invention being defined by the following claims:

What is claimed is:

1. A coaxial joint comprising:

a first cylindrical outer conductor;

a second cylindrical outer conductor concentric with said first outer conductor, said rst and second conductors being relatively rotatable with respect to one another;

means for coupling DC and low frequency signals from one of said outer conductors to the other;

means for coupling signals of higher frequency than said DC and low frequency signals from said one outer conductor to said other outer conductor;

said last two mentioned coupling means being respectively disposed in two separate paths in said outer conductors;

a tirst cylindrical inner conductor;

a second cylindrical inner conductor, said lirst and second inner conductors being relatively rotatable with respect to one another;

means for coupling said DC and low frequency signals from one of said inner conductors to the other;

means for coupling said signals of higher frequency than said DC and low frequency signals from said one inner conductor to said other inner conductor; and

said last two mentioned coupling means being respectively disposed in two separate paths in said inner conductors.

2. A coaxial rotary joint, as in claim 1, Where the electrical ratio of the diameter of said cylindrical inner `conductors to the inner diameter of said cylindrical outer conductors is maintained approximately constant over the entire length of said coaxial rotary joint, thereby facilitating the transfer of said higher lfrequency signals.

3. A coaxial rotary joint, as in claim 2, where said DC and low frequency signals extend from DC to approximately 1000 mc.

4. A coaxial rotary joint, as in claim 3, where said higher frequencies approximately extend from greater than 1000 mc. to 12 gc.

5. A coaxial rotary joint as in claim 1 where said outer conductor, DC and low frequency coupling means comprises a mercury slip ring.

6. A coaxial rotary joint, as in claim 5, where said outer conductor, higher frequency coupling means comprises an air gap disposed between said outer conductors for capacitively coupling said higher frequency signal between said outer conductors.

7. A coaxial rotary joint, as in claim 6, where said inner conductor, DC and low frequency coupling means comprises:

an electrically conductive fluid bath disposed in a recess in said -iirst cylindrical inner conductor and a projection connected to said second cylindrical inner conductor and disposed in said iiuid bath.

8. A coaxial joint, as in claim 7, where said uid is mercury.

9. A coaxial rotary joint, as in claim 7, including means for sealing said electrically conductive nid bath, said sealing means comprising an O-ring mounted around said projection.

10. A coaxial rotary joint, as in claim S, including sealing means for said mercury slip ring, said sealing means including a pair of sealed ball bearing assemblies mounted around and spaced along said first cylindrical outer conductor, said mercury slip ring being disposed between said ball bearing assemblies.I

11. A coaxial rotary joint, as in claim 10, where said sealed ball bearing assemblies respectively contain lubricant which lubricates both the seals of the ball bearing assemblies and the ball bearings.

12. A coaxial rotary joint comprising:

a first cylindrical outer conductor;

a second cylindrical outer conductor concentric with said first conductor, said rst and second conductors :being relatively rotatable with respect to one another;

means for coupling DC and low frequency signals from one of said outer conductors to the other;

means for coupling signals of higher frequency than said DC and low frequency signals from said one outer conductor to said other outer conductor;

said last two mentioned coupling means being respectively disposed in two separate paths in said outer conductors;

a lirst cylindrical inner conductor;

a second cylindrical inner conductor, said iirst and second inner conductors being relatively rotatable with respect to one another; and

means for coupling signals of said higher frequency lfrom one of said inner conductors to the other.

13. A coaxial rotary joint, as in claim 12, where the electrical ratio of the diameter of said cylindrical inner conductors to the inner diameter of said cylindrical outer conductors is maintained approximately constant over the entire length of said coaxial rotary joint, thereby facilitating the transfer of said higher frequency signals.

14. A coaxial rotary joint, as in claim 12, where said outer conductor, DC and low frequency coupling means comprises a mercury slip ring.

15. A coaxial rotary joint, as in claim 14 including sealing means for said mercury slip ring, said sealing means including a pair of sealed ball bearing assemblies mounted around and spaced along said rst cylindrical outer conductor, said mercury slip ring being disposed between said ball bearing assemblies.

References Cited UNITED STATES PATENTS 3,022,479 2/ 1962 Rohrbach 339-8 3,078,432 2/ 1963 Kenyon 339-8 3,107,960 10/ 1963 Neher et al 339-8 3,229,234 1/ 1966 Lattanzi 339-5 RICHARD E. MOORE, Primary Examiner. 

